1. Field of the Invention
The invention is in the field of tension rods for string musical instruments.
2. Description of the Prior Art
The terms "sonic" and "sound" are used herein to mean the complete spectrum of compression wave frequencies including audio frequencies and frequencies above and below the audio range.
It is traditional for string instruments such as guitars to have wooden braces positioned on the underside of the guitar top plate. Generally, a plurality of braces are provided which are positioned in a definite pattern on the instrument during its manufacture in order to improve and enhance the generated sound. In designing the brace pattern, the overall objective has been to reduce localized strain and distortion of the top plate while allowing and enhancing diaphragm movement and resonance.
As is quite apparent in inexpensive guitars, the decay of a note is not uniform, but rather has an undulating or a decreasing and increasing sound pattern, which sometimes will be referred to as "rollover". Rollover is thought to be associated with a feedback between the guitar body and the strings themselves via the bridge and saddle members. In guitars of excellent or outstanding quality, the discretely formed and carefully positioned wood bracings on the inside of the top plate of the guitar have the effect of reducing the rollover to some extent thereby providing a somewhat more uniform sound decay. However, a significant random, uncontrolled, vibratory feedback from the body to the strings still exists, and an undulating sound decay is still present even in the best quality instruments.
The conventional stringed instrument bridge presents an unstable platform for the strings which tends to vibrate with the strings, thereby having a muting effect on the upper partial frequencies, so that the strings may only vibrate in the fundamental and first few harmonics. Thus, the desired full spectrum of overtone frequencies is never produced in the instrument, and the quality of musical sounds produced by the instrument is seriously limited. This also causes an undesirably large proportion of low frequency sonic energy output of the instrument, with consequently generally poor carrying power.
The feedback of a flexural nature which occurs through the bridge of a guitar or other stringed instrument involves an undesired phase modulation which causes a cancellation effect that is detrimental not only to the dynamic amplitude and frequency, but also to the timbre. This undesirable modulation involves a phase shifting not only between the top and back plates of the instrument, but also between the strings and both the top plate and back plate. Timbre is affected in at least two ways: (1) the sine wave component of the sound is distorted, and (2) the formant structure of the tone is altered periodically during the decay.
On string instruments such as violins, it is conventional to utilize a sound post between the inner surfaces of the top and back plates of the instrument. Such sound posts are generally positioned adjacent the treble foot of the bridge and may apply outward force in opposition to the inward force exerted by the taut string members. Such sound posts have not been utilized in guitars, however, inasmuch as the physical dimensions and construction of the guitar have not so required, and it was heretofore thought desirable to maintain a large, relatively free and flexible surface for permitting diaphragm-like motions of the top plate and resulting resonances within the instrument body. Further, a sound post, which would be in compression between the top and back plates, incorporated in a guitar would tend to bow the top plate upwardly, which would enhance the upward biasing produced by the strings to an undesirable level and would correspondingly tend to relieve the string tension. Thus, although wooden braces are utilized in guitars to reduce localized strains and warping of the top plate, the desire for an overall diaphragm-like action has taught away from any utilization of a dampening means directly in the guitar bridge member itself.
In applicants' co-pending application, Ser. No. 533,221, filed Dec. 16, 1974, now U.S. Pat. No. 3,951,031 a solid rigid bridge is utilized in a guitar to reduce rollover and consequently provide a more uniform sound decay. A lead bridge member was found to provide the best dampening capabilities. An alternate solution to the rollover problem is provided by applicants' tension rod described hereinafter.